1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and more particularly, to a control system for an internal combustion engine which includes an injector for cylinder-inside injection to inject fuel into a cylinder at a high load and an injector for intake port injection to inject fuel into an intake port at low and medium loads, and which switches the fuel injection from one of the injectors to the other thereof for use depending on operating conditions.
2. Description of the Related Art
In general, there is known an internal combustion engine that includes an injector for cylinder-inside injection to inject fuel into a cylinder and an injector for intake port injection to inject fuel into an intake port and is adapted to switch from one of these injectors to the other for use depending on operating condition ranges of the engine, which is disclosed in Japanese Patent Application Laid-open No. 63-154816, No. 2002-364409, and No. 6-193496 or the like.
According to a system described in the Japanese Patent Application Laid-open No. 63-154816, in an engine adapted to be operated switching between stratified combustion by direct injection in a low-load operating range and homogeneous combustion by manifold injection in a high-load operating range, at the time of switching between the direct injection and the manifold injection, both injections are performed simultaneously to prevent a degraded ignitability caused by a delay in response or in follow-up to injection switching.
Further, a system described in the Japanese Patent Application Laid-open No. 2002-364409 realizes stratified combustion in a low-load operating range and homogeneous combustion in a high-load operating range in order to improve fuel efficiency and output characteristics, and allows fuel to be injected also by an injector for cylinder-inside injection during an operation under homogeneous combustion by a fuel injection by an injector for intake port injection shifted from an operation under stratified combustion in order to lower the temperature in the cylinder and further the temperature of the injector for cylinder-inside injection by cooling action associated with fuel vaporization, thus preventing deposits from accumulating.
Furthermore, in a system described in the Japanese Patent Application Laid-open No. 6-193496, when a fuel injection is switched from a cylinder-inside injection by an injector for cylinder-inside injection to a manifold injection by an injector for intake port injection, an intake air quantity is decreased, as well as a fuel supply quantity is increased during operation of the injector for cylinder-inside injection before the switching, and then the fuel injection is switched to an active state of the injector for intake port injection, thereby suppressing torque fluctuations. On the other hand, when switching the operation from the injector for intake port injection to the injector for cylinder-inside injection, the system increases the intake air quantity while decreasing the fuel supply quantity after switching the fuel injection from the active state of the injector for intake port injection to an active state of the injector for cylinder-inside injection.
The systems described in the above Japanese Patent Application Laid-open No. 63-154816 and No. 2002-364409 are designed for an engine to be operated switching between the stratified combustion in the low-load operating range and the homogeneous combustion in the high-load operating range where changing the form of fuel supply causes no significant torque fluctuations. Therefore, no reference is made to such torque fluctuations.
Incidentally, in an engine equipped with an injector for cylinder-inside injection to inject fuel into a cylinder at a high load and an injector for intake port injection to inject fuel into an intake port at low and medium loads, when, for example, a fuel injection is switched from the injector for intake port injection at low and medium loads to the injector for cylinder-inside injection, charging efficiency improves, compared to the case with intake port injection, with the aid of latent heat of vaporization of fuel injected by the injector for cylinder-inside injection directly into the cylinder. As a result, there occurs an increase in the intake air quantity, thus causing torque fluctuations (increase). When, on the other hand, the fuel injection is switched from the injector for cylinder-inside injection at a high load to the injector for intake port injection, the charging efficiency decreases, thus causing a change in torque fluctuations (decrease).
Accordingly, in order to suppress such torque fluctuations, application of the technology described in the above Japanese Patent Application Laid-open No. 6-193496 may be considered. However, when the technology described therein is applied to an internal combustion engine which includes an injector for cylinder-inside injection to inject fuel into a cylinder at a high load and an injector for intake port injection to inject fuel into an intake port at low and medium loads, and switches the fuel injection from one of both injectors to the other for use depending on operating conditions, there occurs all the more a problem of producing torque fluctuations at the time of the switching. The reason for this is as follows: when, for example, the fuel injection is switched from the injector for intake port injection at low and medium loads to the injector for cylinder-inside injection, charging efficiency improves, compared to the case with intake port injection, with the aid of latent heat of vaporization of fuel injected by the injector for cylinder-inside injection directly into the cylinder as stated above. As a result, there occurs a change in the intake air quantity, thus causing torque fluctuations.